Glass fibers are produced as follows: glass raw materials mixed so as to give a glass composition having a desired composition were melted in a melting furnace to prepare molten glass (melt of glass composition), the molten glass is discharged from a vessel having a nozzle plate in which tens to thousands of nozzle tips are formed (called “bushing”), the discharged molten glass is taken up at a high speed and thus cooled while being stretched, and thus solidified into fiber shapes (hereinafter, this operation is sometimes referred to as “spinning”). The bushing is formed of, for example, a noble metal such as platinum.
As a glass for forming the glass fiber, there has hitherto been known a glass having a composition composed of SiO2, Al2O3 and MgO (S-glass). According to S-glass, glass fibers having high fiber strength and high fiber modulus of elasticity can be obtained as compared with general-purpose glass such as E-glass; however S-glass is problematic in that the spinning of glass fiber is not necessarily easy in view of the 1000-poise temperature and the liquidus temperature.
The 1000-poise temperature as referred to herein means the temperature at which the viscosity of the molten glass comes to be 1000 poises (100 Pa·s), and the liquidus temperature as referred to herein means the temperature at which crystals start to precipitate while the temperature of the molten glass is being lowered. In general, glass fibers can be efficiently spun when the viscosity of the molten glass is allowed to be in the vicinity of 1000 poises. Accordingly, usually the wider the temperature range between the 1000-poise temperature and the liquidus temperature (working temperature range), the more easily and the more stably the spinning of glass fibers can be performed.
In S-glass, the 1000-poise temperature and the liquidus temperature are extremely close to each other, and the working temperature range is narrow; accordingly, molten S-glass tends to undergo crystallization (devitrification) even under the effect of slight temperature decrease. Accordingly, in order to stably perform the spinning of S-glass, it is necessary to accurately control the spinning conditions in the production process of glass fibers.
Thus, as a glass composition for glass fiber, produced more easily than S-glass and having a higher fiber strength and a higher fiber modulus of elasticity than the general-purpose glass, the present applicant has proposed a glass composition having the composition in which, in relation to the total amount of the glass composition, the content of SiO2 is 57.0 to 63.0% by mass, the content of Al2O3 is 19.0 to 23.0% by mass, the content of MgO is 10.0 to 15.0% by mass, the content of CaO is 4.0 to 11.0% by mass, and the sum content of SiO2, Al2O3, MgO and CaO is 99.5% by mass or more (see Patent Literature 1).
According to the glass composition described in Patent Literature 1, the 1000-poise temperature and the liquidus temperature can be lowered, and it is possible to easily produce glass fibers having a higher fiber strength and a higher fiber modulus of elasticity than the general-purpose glass, by spinning the molten glass prepared by melting the glass raw materials mixed so as to give the glass composition having the composition falling within the foregoing ranges.
In the spinning of the molten glass prepared by melting the glass raw materials mixed so as to give the glass composition having the composition falling within the range described in Patent Literature 1, a bushing provided with circular nozzle tips is usually used, and the bushing is set at a temperature higher than the liquidus temperature of the molten glass. In this way, the molten glass discharged from the circular nozzle tips has a strong effect to become round due to the surface tension of itself, and thus it is possible to easily obtain glass fibers having cross sectional shapes close to a perfect circle. When the viscosity of the molten glass is 200 poises (20 Pa·s) or less, the glass discharged from the nozzle tips takes droplet shapes without forming fibers. Accordingly, the bushing is controlled at a temperature equal to or lower than the temperature at which the viscosity of the molten glass is 200 poises.
When glass fibers having non-circular cross sections such as flat shaped glass fibers are obtained, a bushing provided with flat-shaped nozzle tips is used. When a molten glass is spun at a viscosity of 1000 poises or less, the viscosity of the glass discharged from the nozzle tips is too low, and the discharged glass becomes round due to the surface tension of itself so as to make it difficult to obtain glass fibers having non-circular cross sections. Accordingly, a bushing provided with flat-shaped nozzle tips is controlled at a temperature which is higher than the liquidus temperature of the molten glass and at which the viscosity of the molten glass is 1000 poises or more. In this way, the viscosity of the molten glass discharged from the flat-shaped nozzle tips is heightened, the surface tension of the molten glass itself is made difficult to function, and thus it is made possible to obtain glass fibers having non-circular cross sections such as flat shaped glass fibers in accordance with the shapes of the openings of the nozzle tips.